On Wednesday 11 March 2009 01:02:38 Andy Walls wrote:
> On Mon, 2009-03-09 at 22:53 +0100, Hans Verkuil wrote:
> > On Monday 09 March 2009 21:47:55 Andy Walls wrote:
> >
> > I'm almost done the spec
> >
> > > changes (see the diff inline below), but I can easily change things.
> >
> > Nice documentation!
>
> Thanks! When I'm motivated, for some mysterious reason, to do some
> documentation, I try to be thorough.
>
>
> The linemask[2] bitmask specifications are giving me some mild
> headaches. The linemaks output by the cx18 driver code for NTSC CC
> don't seem to be consistent with README.vbi:
>
> xxd -g4 foovbi.mpg | less
>
> 0000800: 000001ba 44000c66 240101d1 d3faffff ....D..f$.......
> 0000810: 000001bd 00428480 07210001 0001ffff .....B...!......
> 0000820: 69747630 00800000 00000000 04c146ad itv0..........F.
> 0000830: 107f1080 10821081 10841081 10851084 ................
> 0000840: 10811083 10711063 10541023 10471010 .....q.c.T.#.G..
> 0000850: 105a1022 10781004 000001ba 44000404 .Z.".x......D...
> [...]
> 001a850: 84e015f9 24c26e47 000001ba 44000c66 ....$.nG....D..f
> 001a860: 240101d1 d3faffff 000001bd 006e8480 $............n..
> 001a870: 07210001 0001ffff 69747630 00800000 .!......itv0....
> 001a880: 02000000 04808066 1082107f 1080107e .......f.......~
> 001a890: 107c107f 107b1081 107d1080 107f107f .|...{...}......
> 001a8a0: 107f1080 10801082 10831081 10831004 ................
> 001a8b0: 85027110 86107f10 85107e10 82107d10 ..q.......~...}.
> 001a8c0: 7f107f10 81107f10 83107f10 83107f10 ................
> 001a8d0: 84107f10 84107e10 82100000 000001ba ......~.........
> [...]
> 00210d0: 03db2840 76591671 c3eb4ec1 000001ba ..(@vY.q..N.....
> 00210e0: 44000c66 240101d1 d3faffff 000001bd D..f$...........
> 00210f0: 006e8480 07210001 0001ffff 69747630 .n...!......itv0
> 0021100: 00800000 02000000 04808066 107f1076 ...........f...v
> 0021110: 107c107d 107f1082 10801080 10811081 .|.}............
> 0021120: 108f109e 1fa810db e7b610f0 10a910d8 ................
> 0021130: 10911004 20206a10 83108110 83107f10 .... j.........
> 0021140: 83107e10 81107d10 7d107c10 7c107d10 ..~...}.}.|.|.}.
> 0021150: 7d107f10 7d108110 7d108210 7e100000 }...}...}...~...
>
> (Ignore the PTS of 0 in the MPEG-2 Private Stream 1 PES header, I know
> about that.)
>
> Or maybe it's that the linemasks are written to the file as little
> endian, when the rest of the Program Pack and Private Stream 1 Packet
> are written big-endian or bytewise.
>
> So I guess should be reading the linemasks as 0x00008000 (line 6+15 =
> 21) and 0x00000002 (line 6+(32-18)+1 = 21).
>
> Should the linemask[] values be specified as little endian in the output
> stream?
Yes, that must definitely be specified. And while you are at it, ivtv and
probably cx18 as well don't handle the endianness of linemask when writing
the VBI data, so that should be fixed. It will currently fail with a
big-endian system.
Regards,
Hans
>
>
> MythTV has this odd input method for the linemask[] values:
>
> void AvFormatDecoder::ProcessVBIDataPacket(
> const AVStream *stream, const AVPacket *pkt)
> {
> (void) stream;
>
> const uint8_t *buf = pkt->data;
> uint64_t linemask = 0;
> unsigned long long utc = lastccptsu;
>
> // [i]tv0 means there is a linemask
> // [I]TV0 means there is no linemask and all lines are present
> if ((buf[0]=='t') && (buf[1]=='v') && (buf[2] == '0'))
> {
> /// TODO this is almost certainly not endian safe....
> memcpy(&linemask, buf + 3, 8);
> buf += 11;
> }
> else if ((buf[0]=='T') && (buf[1]=='V') && (buf[2] == '0'))
> {
> linemask = 0xffffffffffffffffLL;
> buf += 3;
> }
> [...]
>
> So if we say in the spec the linemask[] values must be little endian,
> MythTV will continue to work on little endian machines (I think).
>
> Regards,
> Andy
>
> > I think that when this goes in, then README.vbi can be
> > removed.
> >
> > Regards,
> >
> > Hans
--
Hans Verkuil - video4linux developer - sponsored by TANDBERG
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